Separation of m-xylene from o-xylene by extractive distillation

ABSTRACT

m-Xylene is difficult to separate from o-xylene by conventional rectification or distillation because of the close proximity of their boiling points. m-Xylene can be readily separated from o-xylene by using extractive distillation in which the extractive agent comprises propoxypropanol; propoxypropanol and 1,4-butanediol; ethyl benzoate and ethylene glycol phenyl ether and benzyl alcohol.

DUAL APPLICATION

This application is related to application Ser. No. 06/599,277 filedApr. 11, 1984, now U.S. Pat. No. 4,488,937, by Lloyd Berg and An-I Yehwhich also is directed to the separation of xylenes by extractivedistillation using other agents.

FIELD OF THE INVENTION

This invention relates to a method for separating m-xylene from o-xyleneusing individual or mixtures of two or more organic compounds asextractive agents in extractive distillation.

DESCRIPTION OF THE PRIOR ART

Extractive distillation is the method of separating close boilingcompounds by carrying out the distillation in a multi-platerectification column in the presence of an added liquid or liquidmixture, said liquid(s) having a boiling point higher than the compoundsbeing separated. The extractive agent is introduced near the top of thecolumn and flows downward until it reaches the stillpot or reboiler. Itspresence on each plate of the rectification column alters the relativevolatility of the close boiling compounds in a direction to make theseparation on each plate greater and thus require either fewer plates toeffect the same separation or make possible a greater degree ofseparation with the same number of plates. The extractive agent shouldboil higher than any of the close boiling liquids being separated andnot form minimum boiling azeotropes with them. Usually the extractiveagent is introduced a few plates from the top of the column to insurethat none of the extractive agent is carried over with the lowestboiling component. This usually requires that the extractive agent boiltwenty Centigrade degrees or more higher than the lowest boilingcomponent.

At the bottom of a continuous column, the less volatile components ofthe close boiling mixtures and the extractive agent are continuouslyremoved from the column. The usual methods of separation of these twocomponents are the use of another rectification column, cooling andphase separation, or solvent extraction.

The operation to separate benzene from close boiling non-aromatichydrocarbons has been well described by Butler, U.S. Pat. No. 3,114,783.He suggests a large number of pure compounds including alcohols, glycolethers and sulfolanes to separate both benzene and toluene. Noinformation is given here on the relative volatility and thus relativeperformance of these compounds as extractive distillation agents. Atlaniet al, French Pat. No. 2,335,584, July 15, 1977 describes the use ofseveral cyanamide derivatives as extractive agents for separatingaromatics including benzene from naphthenes and dienes Cooper, U.S. Pat.No. 2,655,467 employs molten phthalic anhydride as the extractivedistillation agent to separate aromatics including benzene fromnon-aromatic hydrocarbons. P. Mikitinko, G. Cohen and L. Asselinieau,German Pat. No. 2,313,603, Sept. 27, 1973, separated both benzene andtoluene from non-aromatic hydrocarbons using dimethyl formamide anddimethyl acetamide. P. Mikitinko and L. Asselinieau in German Pat. No.2,809,985, Sept. 14, 1978, use these same reagents with water added tobring the non-aromatic hydrocarbons off overhead as a two-phaseazeotrope and thus lower the boiling point, K. Eisenlohr and H. Muellerin German Pat. No. 2,263,344, Dec. 23, 1972 reported on an improvedequipment arrangement to separate both benzene and toluene fromnon-aromatic hydrocarbons by extractive distillation. G. Preusser, M.Schulze, K. Richter and W. Heuwels in German Pat. No. 1,960,857, Dec. 4,1969 described the use of morpholine and some of its derivatives forthis separation. Improved equipment for this separation was presented byE. Mueller and K. P. John in German Pat. No. 1,808,758, Nov. 14, 1968.L. Berg, U.S. Pat. No. 4,363,704 described the separation of toluenefrom close boiling non-aromatic hydrocarbons. The separation of isomersby extractive distillation has received considerably less attention. L.Berg, U.S. Pat. Nos. 4,292,142 and 4,299,668 described the use of chlorocompounds and oxygenated compounds to separate ethylbenzene fromp-xylene and m-xylene. Prior to this invention, nothing has beenreported in the separation by extractive distillation of one xylene fromanother xylene.

The advantage of using extractive distillation in this separation can beseen from Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Theoretical and Actual Plates Required vs. Relative Volatility                for m-Xylene - o-Xylene Separation.                                           Relative                                                                              Theoretical Plates Required                                                                     Actual Plates                                       Volatility                                                                            at Total Reflux, 95% Purity                                                                     Required, 75% Eff.                                  ______________________________________                                        1.12    52                70                                                  1.20    33                44                                                  1.25    27                36                                                  1.30    23                31                                                  1.35    20                27                                                  1.40    18                24                                                  ______________________________________                                    

The relative volatility of m-xylene to o-xylene is about 1.12. Toseparate these two by conventional rectification requires a minimum of52 theoretical plates. This however is at total reflux. At a specificreflux, it will be more. The theoretical plates have to be converted toactual plates. Plate efficiencies of 75% are commonly employed and thisis the basis of the actual plate listing in Table 1. Thus more than 70actual plates are required, clearly a difficult separation. Severalextractive distillation agents that we have discovered push the relativevolatility as high as 1.4 and Table 1 shows that they will reduce theactual plate requirement to something close to 24 plates. Converting thetotal reflux to an actual reflux will increase the plate requirementsomewhat but still make for a very attractive separation.

Extractive distillation typically requires the addition of an equalamount to twice as much extractive agent as close boiling compounds oneach plate of the rectification column. The extractive distillationagent should be heated to about the same temperature as the plate intowhich it is introduced. Thus extractive distillation imposes anadditional heat requirement on the column as well as somewhat largerplates for the same product output. To be economically attractive, theextractive distillation system must save more in the reduction of thenumber of theoretical plates and the size of the column than it adds inthe cost of larger plates and the additional heat requirement. This willvary depending on the difficulty of the separation and the cost of heat.We found that in the separation of m-xylene from o-xylene, theextractive agent should increase the relative volatility to about 1.3 tomake the process economically attractive under the equipment and heatcosts in effect at the time of our investigation.

Another consideration in the selection of the extractive distillationagent is its recovery from the bottoms product. The usual method is byrectification in another column. In order to keep the cost of thisoperation to a minimum, an appreciable boiling point difference betweenthe compound being separated and the extractive agent is desirable. Werecommend twenty Centigrade degrees or more difference.

The xylenes are major precursors to many processes for making plasticsand dyes. In these uses it is absolutely essential that the xylenes bevery pure. It is the presence of impurities that make them poorpolymerizing agents as a plastic or render them inconistent as dyeintermediates. The xylenes of commerce originate either from coal tar orfrom petroleum, usually via the hydroforming of the correspondingnaphthenes and thus are always found as mixtures of isomers.

OBJECTIVE OF THE INVENTION

The object of this invention is to provide a process or method ofextractive distillation that will enhance the apparent relativevolatility of m-xylene to o-xylene in their separation in arectification column. It is a particular object of this invention toidentify suitable combinations of organic compounds which will increasethe apparent relative volatility of m-xylene to o-xylene to valueshigher than 1.3. It is a further object of this invention to identifymixtures of organic compounds which, in addition to the aboveconstraints, are stable, can be separated from o-xylene by rectificationwith relatively few actual plates and can be recycled to the extractivedistillation column and reused with little decomposition.

SUMMARY OF THE INVENTION

The objects of this invention are provided by a process for theseparation on m-xylene from o-xylene using sulfolane, benzoates,dimethylformamide, propoxypropanol and/or oxygenated, nitrogenous orsulfur containing organic compounds as the agent in extractivedistillation.

DETAILED DESCRIPTION OF THE INVENTION

We have discovered that benzoates, dimethylformamide andpropoxypropanol, singly but principally in mixtures will enhance therelative volatility of m-xylene to o-xylene. m-Xylene (b.p.=139.2° C.)and o-xylene (b.p.=144.5° C.) have a relative volatility of 1.12. Wehave discovered a number of extractive distillation agents which willenhance the relative volatility of these two to the 1.3 to 1.4 range.

Table 2 shows a number of organic compounds which when mixed withvarious benzoates, are excellent extractive distillation agents for thisseparation. Table 3 shows that dimethylformamide (DMFA) either alone ormixed with certain organic compounds, is an effective agent. Table 4shows that propoxypropanol mixed with certain organic compounds, is aneffective agent in this separation.

All of the systems in Tables 2, 3, and 4 possess a relative volatilityof 1.3 or greater. The relative volatilities shown in Table 2, 3, and 4are the average of two runs, one at one part of extractive agent perpart of xylene mixture and the other at 6/5 parts of extractive agentper part of xylene mixture. We have found that this is the preferredratio of extractive distillation agent to xylenes in this separation.The amount of each compound in the binarys and ternarys wasapproximately equal to each other. The exact ratio does not appear to becritical. Likewise the relative volatilities shown in Tables 2, 3, and 4do not change appreciably when the ratio of m-xylene to o-xylene isvaried. The data presented in Tables 2, 3, and 4 were obtained in aglass vapor-liquid equilibrium still of the Othmer design.

Several of the agents listed in Table 2-4 and whose relativevolatilities had been determined in the vapor-liquid equilibrium still,were then evaluated in a glass perforated plate rectification columnpossessing 4.5 theoretical plates and a glass column packed with Berlsaddles possessing 2.4 theoretical plates.

                                      TABLE 2                                     __________________________________________________________________________    Relative Volatilities of m-Xylene - o-Xylene With Mixtures Containing         Benzoates.                                                                                                        Relative                                  Extractive Distillation Agent       Volatility                                __________________________________________________________________________    Methyl benzoate, Butyl benzoate     1.33                                      Methyl benzoate, Ethyl benzoate     1.34                                      Methyl benzoate, Diisodecyl phthalate                                                                             1.30                                      Methyl benzoate, Diphenyl isophthalate                                                                            1.35                                      Methyl benzoate, DMSO, Butyl benzoate                                                                             1.38                                      Methyl benzoate, DMSO, Ethyl benzoate                                                                             1.33                                      Ethyl benzoate, Diphenyl terephthalate                                                                            1.31                                      Ethyl benzoate, Ethylene glycol phenyl ether                                                                      1.33                                      Ethyl benzoate, Dimethyl phthalate  1.30                                      Ethyl benzoate, DMSO, Butyl benzoate                                                                              1.31                                      Ethyl benzoate, Ethylene glycol phenyl ether, Methyl salicylate                                                   1.31                                      Ethyl benzoate, Ethylene glycol phenyl ether, Benzyl alcohol                                                      1.30                                      Ethyl benzoate, Butyl benzoate, Benzyl butyl phthalate                                                            1.31                                      Butyl benzoate                      1.33                                      Butyl benzoate, Diphenyl isophthalate                                                                             1.35                                      Benzyl benzoate, Benzyl butyl phthalate                                                                           1.32                                      Benzyl benzoate, Ethyl - 4-hydroxybenzoate                                                                        1.33                                      Benzyl benzoate, Methyl - 4-hydroxybenzoate                                                                       1.31                                      Benzyl benzoate, Ethyl - 2-hydroxybenzoate                                                                        1.39                                      Benzyl benzoate, Methyl - 2-hydroxybenzoate                                                                       1.38                                      Benzyl benzoate, Butyl benzoate     1.31                                      Benzyl benzoate, Benzoic acid       1.32                                      Benzyl benzoate, DMSO, Methyl - 2-hydroxybenzoate                                                                 1.42                                      Benzyl benzoate, DMSO, Ethyl - 2-hydroxybenzoate                                                                  1.33                                      Benzyl benzoate, DMSO, Methyl benzoate                                                                            1.34                                      Benzyl benzoate, DMSO, Ethyl benzoate                                                                             1.35                                      Benzyl benzoate, DMSO, Butyl benzoate                                                                             1.34                                      Benzyl benzoate, DMSO, Benzoic acid 1.32                                      Methyl - 2-hydroxybenzoate          1.39                                      Methyl - 2-hydroxybenzoate, Diisononyl phthalate                                                                  1.32                                      Methyl - 2-hydroxybenzoate, Butyl benzoate                                                                        1.32                                      Methyl - 2-hydroxybenzoate, Benzyl benzoate                                                                       1.37                                      Methyl - 2-hydroxybenzoate, Methyl benzoate                                                                       1.30                                      Methyl - 2-hydroxybenzoate, DMSO, Butyl benzoate                                                                  1.34                                      Methyl - 2-hydroxybenzoate, DMSO, Ethyl benzoate                                                                  1.33                                      Methyl - 2-hydroxybenzoate, DMSO, Methyl benzoate                                                                 1.37                                      Butyl - 4-hydroxybenzoate, bis-Phenol A                                                                           1.34                                      Ethyl - 2-hydroxybenzoate           1.34                                      Ethyl - 2-hydroxybenzoate, Methyl benzoate                                                                        1.35                                      Ethyl - 2-hydroxybenzoate, Butyl benzoate                                                                         1.33                                      Ethyl - 2-hydroxybenzoate, Methyl - 2-hydroxybenzoate                                                             1.36                                      Ethyl - 2-hydroxybenzoate, Ethyl benzoate                                                                         1.30                                      Ethyl - 2-hydroxybenzoate, DMSO, Methyl benzoate                                                                  1.37                                      Ethyl - 2-hydroxybenzoate, DMSO, Ethyl benzoate                                                                   1.32                                      Ethyl - 2-hydroxybenzoate, DMSO, Butyl benzoate                                                                   1.35                                      Ethyl - 2-hydroxybenzoate, DMSO, Methyl - 2-hydroxybenzoate                                                       1.31                                      Ethyl - 2-hydroxybenzoate, Diphenyl isophthalate                                                                  1.32                                      Diethylene glycol dibenzoate        1.30                                      Diethylene glycol dibenzoate, Methyl benzoate                                                                     1.34                                      Diethylene glycol dibenzoate, Ethyl benzoate                                                                      1.36                                      Diethylene glycol dibenzoate, Benzyl benzoate                                                                     1.35                                      Diethylene glycol dibenzoate, Butyl benzoate                                                                      1.30                                      Diethylene glycol dibenzoate, Methyl - 2-hydroxybenzoate                                                          1.31                                      Diethylene glycol dibenzoate, Ethyl - 2-hydroxybenzoate                                                           1.39                                      Dipropylene glycol dibenzoate (DPGDB)                                                                             1.33                                      DPGDB, Diphenyl isophthalate        1.34                                      DPGDB, Ethyl benzoate               1.39                                      DPGDB, Benzyl benzoate              1.30                                      DPGDB, Methyl benzoate              1.31                                      DPGDB, Ethyl - 2-hydroxybenzoate    1.30                                      DPGDB, Diethylene glycol dibenzoate 1.32                                      DPGDB, Methyl - 2-hydroxybenzoate   1.40                                      DPGDB, DMSO                         1.38                                      DPGDB, DMSO, Benzyl benzoate        1.33                                      DPGDB, DMSO, Methyl benzoate        1.39                                      DPGDB, DMSO, Methyl - 2-hydroxybenzoate                                                                           1.40                                      DPGDB, DMSO, Ethyl - 2-hydroxybenzoate                                                                            1.36                                      DPGDB, DMSO, Butyl benzoate         1.31                                      DPGDB, DMSO, Butyl benzyl phthalate 1.33                                      DPGDB, Methyl benzoate, Diphenyl isophthalate                                                                     1.39                                      DPGDB, Methyl benzoate, Butyl - 4-hydroxybenzoate                                                                 1.30                                      DPGDB, Ethyl benzoate, Adiponitrile 1.32                                      DPGDB, Diethylene glycol dibenzoate, Methyl - 2-hydroxybenzoate                                                   1.35                                      DPGDB, Diethylene glycol dibenzoate, Methyl - 2-hydroxybenzoate,                                                  1.33                                      DPGDB, Diethylene glycol dibenzoate, Ethyl - 2-hydroxybenzoate                                                    1.31                                      DPGDB, Diethylene glycol dibenzoate, Ethyl - 2-hydroxybenzoate,                                                   1.34                                      DPGDB, Diphenyl isophthalate, Methyl - 2-hydroxybenzoate                                                          1.35                                      DPGDB, Diphenyl isophthalate, Ethyl - 2-hydroxybenzoate                                                           1.37                                      __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                        Relative Volatilities of m-Xylene - o-Xylene With Mixtures                    Containing Dimethylformamide (DMFA).                                                                      Relative                                          Extractive Distillation Agent                                                                             Volatility                                        ______________________________________                                        Dimethylformamide (DMFA)    1.33                                              Dimethylformamide (DMFA), Ethylene glycol                                                                 1.34                                              Dimethylformamide (DMFA), 1,4-Butanediol                                                                  1.32                                              Dimethylformamide (DMFA), 1,5-Pentanediol                                                                 1.31                                              Dimethylformamide (DMFA), Diethylene glycol                                                               1.32                                              Dimethylformamide (DMFA), Triethylene glycol                                                              1.32                                              Dimethylformamide (DMFA), Ethylene glycol phenyl                              ether                                                                         Dimethylformamide (DMFA), Adiponitrile,                                                                   1.35                                              1,4-Butanediol                                                                Dimethylformamide (DMFA), Adiponitrile,                                                                   1.34                                              Dihexyl phthalate                                                             Dimethylformamide (DMFA), Hydroquinone,                                                                   1.33                                              m-Cresol                                                                      Dimethylformamide (DMFA), Hydroquinone,                                                                   1.33                                              p-Cresol                                                                      ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Relative Volatilities of Mixtures Containing Propoxypropanol.                                        Relative                                               Extractive Distillation Agent                                                                        Volatility                                             ______________________________________                                        Propoxypropanol, 1,4-Butanediol                                                                      1.35                                                   Propoxypropanol, Propylene glycol                                                                    1.35                                                   Propoxypropanol, 1,5-Pentanediol                                                                     1.31                                                   Propoxypropanol, Hexylene glycol                                                                     1.35                                                   Propoxypropanol, Diethylene glycol                                                                   1.38                                                   ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Data From Runs Made In Rectification Columns.                                          Column  Wt. % of m-Xylene                                                                            Relative                                      Agent      Type      Overhead  Bottoms                                                                              Volatility                              ______________________________________                                        Methyl benzoate                                                                          Perforated                                                                              51.2      31.1   1.21                                               plate                                                              Ethyl benzoate                                                                           Perforated                                                                              50.0      30.6   1.20                                               plate                                                              Benzyl benzoate                                                                          Perforated                                                                              50.7      27.8   1.24                                               plate                                                              Methyl benzoate                                                                          Packed    40.7      28.4   1.26                                    Ethyl benzoate                                                                           Packed    41.6      28.1   1.29                                    ______________________________________                                         Notes:                                                                        The feed mixture was 150 gr. mxylene and 350 gr. oxylene. The agent was       added at a rate of 20 ml/min. and 65° C. Total reflux was 5-10         ml/min.                                                                  

THE USEFULNESS OF THE INVENTION

The usefulness or utility of this invention can be demonstrated byreferring to the data presented in Table 2, 3, 4, and 5. When m-xyleneis being separated from o-xylene, relative volatility 1.12, byrectification in 95% purity, Table 1 shows that about 70 actual platesare required. Table 2 shows that a mixture of methyl benzoate anddiphenyl isophthalate changes the relative volatility to 1.35 andreferring to Table 1, this requires only 27 actual plates. Table 2 showsthat when the extractive distillation agent is a mixture of dipropyleneglycol dibenzoate and methyl-2-hydroxybenzoate, the relative volatilitybecomes 1.40 and from Table 1, only 24 actual plates will be required toseparate m-xylene from o-xylene in 95% purity.

WORKING EXAMPLES EXAMPLE 1

A mixture comprising 50 grams of m-xylene and 50 grams of o-xylene wascharged to an Othmer type vapor-liquid equilibrium still and the mixturerefluxed for twelve hours. Samples of vapor and liquid were removed andanalysed by gas chromatography. The vapor contained 51.8% m-xylene,48.2% o-xylene, the liquid 49% m-xylene, 51% o-xylene. This indicates arelative volatility of m-xylene to o-xylene of 1.12.

EXAMPLE 2

A mixture comprising 25 grams of m-xylene, 25 grams of o-xylene and 50grams of ethyl-2-hydroxybenzoate was charged to the vapor-liquidequilibrium still used in Example 1 and refluxed for eleven hours.Analyses indicated a vapor composition of 56.3% m-xylene, 43.7% m-xyleneand a liquid composition of 48.5% m-xylene, 51.5% o-xylene. Thisindicates a relative volatility of 1.37. Ten grams ofethyl-2-hydroxybenzoate was added and refluxing continued for ninehours. Analyses gave a vapor composition of 55% m-xylene, 45% o-xylene,a liquid composition of 48.1% m-xylene, 51.9% o-xylene which is arelative volatility of 1.32.

EXAMPLE 3

A mixture comprising 25 grams of m-xylene, 25 grams of o-xylene, 25grams of propoxypropanol and 25 grams of 1,4-butanediol was charged tothe vapor-liquid equilibrium still and refluxed for thirteen hours.Analyses indicated a vapor composition of 47.9% m-xylene, 52.1% o-xyleneand a liquid composition of 40% m-xylene, 60% o-xylene. This indicates arelative volatility of 1.38. Five grams each of propoxypropanol and1,4-butanediol was added and refluxing continued for another elevenhours. Analyses indicated a vapor composition of 54% m-xylene, 46%o-xylene, a liquid composition of 47% m-xylene, 53% o-xylene which is arelative volatility of 1.32.

EXAMPLE 4

A mixture comprising 25 grams of m-xylene, 25 grams of o-xylene, 17grams of sulfolane, 17 grams of DMSO and 17 grams of ethyl benzoate wascharged to the vapor-liquid equilibrium still and refluxed for fifteenhours. Analyses indicated a vapor composition of 54.8% m-xylene, 45.2%o-xylene and a liquid composition of 47.3% m-xylene, 52.7% o-xylene.This indicates a relative volatility of 1.35. Three grams each ofsulfolane, DMSO and ethyl benzoate was added and refluxing continued foranother eleven hours. Analyses indicated a vapor compositon of 54%m-xylene, 46% o-xylene, a liquid composition of 46.7% m-xylene, 53.3%o-xylene which is a relative volatility of 1.34.

These examples serve to show in detail how the data presented in Tables2, 3, and 4 were obtained. Each of the solvent combinations reportedthere was determined in this manner.

EXAMPLE 5

A column consisting of one five plate section of 11/2" diameter glassperforated plates equipped with a vacuum jacket was employed. The columnwas fitted with a Corad head constant reflux ratio distilling head.Between the Corad head and the top of the column, a feed line from aconstant flow bellows pump was introduced. The column had beencalibrated with a test mixture of ethylbenzene and p-xylene, whichmixture possesses a relative volatility of 1.06. The column wascalibrated as 4.5 theoretical plates at total reflux. A run was madewith a charge comprising approximately 150 grams of m-xylene and 350grams of o-xylene in the stillpot. The column was operated at totalreflux for about an hour and then the pump started at a rate to deliverone part of extractive agent to about two parts of xylenes being boiledup. The extractive agent in this example was benzyl benzoate. The dataobtained is presented in Table 5 and shows an overhead analyses of 50.7%m-xylene, a bottoms analysis of 27.8% m-xylene which is a relativevolatility of 1.24. Without the extractive agent, it would have been1.12.

EXAMPLE 6

A glass column consisting of five inches of Berl saddles as packing andhaving been calibrated with ethylbenzene-p-xylene to give 2.4theoretical plates was substituted for the column in Example 5. Theextractive distillation agent was ethyl benzoate and operation wassimilar to Example 5. After 1.5 hours of operation, analyses showed anoverhead composition of 41.6% m-xylene, a bottoms composition of 28.1%m-xylene which is a relative volatility of 1.29. Without the extractiveagent, it would have been 1.12.

The nature of the present invention having been described andillustrated by examples, what we wish to claim as new and useful andsecure by Letters Patent is:
 1. A method for recovering m-xylene from amixture of m-xylene and o-xylene which comprises distilling a mixture ofm-xylene and o-xylene in a rectification column in the presence of anextractive agent, recovering essentially pure m-xylene as overheadproduct and obtaining the extractive agent and o-xylene from thestillpot or reboiler, the extractive agent comprises propoxypropanol. 2.The method of claim 1 in which the extractive agent comprises a mixtureof propoxypropanol and at least one of the group consisting essentiallyof: propylene glycol, 1,4-butanediol, 1,5-pentanediol, hexylene glycol,diethylene glycol.